Process for producing swollen foods

ABSTRACT

Disclosed is a process for producing swollen foods having a softer mouth feel than might be expected from those obtained by a conventional process, which process comprises heating and pressurizing through an extruder a powder; e.g. eggshell or bone powder, comprising fine particles of porous structure and a food material predominantly comprising a starchy material and/or a protein and thereafter releasing and causing the same to swell.

BACKGROUND OF THE INVENTION

The present invention relates to a novel process for producing puffed,inflated, or swollen foods (hereinafter referred to as "swollen"food(s)) and to a specific article of swollen food.

Various attempts have so far been made to obtain swollen foods byheating and pressurizing starchy materials such as flour and cornstarchor proteinous materials such as soybean protein through an extruder andthen swelling the same by sudden release of pressure.

While swollen foods can be produced by such conventional methodsgenerally with high efficiency, the swollen foods obtained heretoforehave generally been liable to have coarse swollen cells or foam bubblesformed by foaming although depending upon the compositions of the foods.When the swollen cells or foam bubbles are coarse, i.e., large, the cellmembranes are generally thick so that the whole swollen food becomeshard, resulting in a hard mouth feel. Consequently, such swollen foodshave had the problem, for example, of being unsuitable for infant foodssuch as baby foods.

Further, in one specific mode of the above method for producing swollenfoods in which a food material predominantly comprising a starchymaterial such as flour is mainly used, the total water content in thestarting materials fed to an extruder is maintained at about 40% and theextruder used has a die outlet with a square cross section, the swellingpercentage of an extrudate at the die outlet is low since the watercontent is somewhat higher than in starting materials ordinarily used,so that an extrudate having a square cross section approaching that ofthe die can be obtained. This extrudate, for example, is then cut to anappropriate length, and water contained therein in excess is removed bydrying, whereby it can be expected that crouton-like articles can beobtained as the end product.

Although, when sprinkled over hot soup, the swollen food thus prepareddoes not soften (or become "mushy") in a time period as short as about10 to 30 seconds, it is not crispy and feels so hard in the mouth thatit is far from being a crouton from the standpoint of textural feel inthe mouth.

Conventional croutons prepared by dicing bread into cubes and drying theso diced bread, on the other hand, can be easily crushed when they areeaten directly but, when sprinkled over hot soup, soften in about 20 to30 seconds and do not feel crispy, thus losing somewhat their pleasantmouth feel.

DESCRIPTION OF THE INVENTION

In view of the above described situation, the primary and basic objectof the present invention is to provide a novel process for producingswollen foods by heating, pressurizing and then swelling through anextruder a food material predominantly comprising a starchy material ora protein, which process ensures the production of swollen foods havinga softer mouth feel than might be expected from those obtained by aconventional process.

A second and subsidiary object of the invention is to provide a novelprocess for producing swollen foods by heating, pressurizing and thenswelling through an extruder a food material predominantly comprising astarchy material, which process ensures the production of swollen foodswhich, when sprinkled over hot soup, do not soften in a time period asshort as about 10 to 30 seconds, i.e., do not lose crispiness in a shorttime period, and yet can be easily crushed (with the teeth), i.e., donot remain too hard.

A third object of the invention is to provide novel swollen foods havingthe desirable properties of the swollen foods produced by the processdescribed with reference to the second object.

As a result of extensive research efforts expended toward the aboveobjects, we have arrived at the present invention.

Basically, the characteristic feature of the process of the presentinvention is to heat, pressurize and thereafter swell through anextruder a powder comprising fine particles of porous structure and afood material predominantly comprising a starchy material and/or aprotein.

DETAILED DESCRIPTION OF THE INVENTION

The term "powder comprising fine particles of porous structure" as usedherein refers to a powder in the form of fine particles containingnumerous micropores, typical examples being powders obtained bypulverization of eggshells, animal bones and the like.

These powders comprise fine particles with a maximum particle size ofpreferably 100 μ or less, and more preferably 50 μ or less. Specificexamples of such powders are those comprising particles with a maximumparticle size of 20 μ or less or in the range of from about 2 to 6 μ,for example, because the smaller the particle size, the less rough mouthfeel will the end product have.

The phrase "food material predominantly comprising a starchy material"is intended herein to mean a food comprising, as the principalingredient thereof, a starch component such as rice, wheat or corn andalso to include starches obtained by extracting only starch componentsfrom cereals such as rice and wheat. Further, the phrase "food materialpredominantly comprising a protein" as used herein designates a foodcomprising, as the principal ingredient thereof, a protein componentsuch as soybeans or soybean protein. While these food materials may bein any form, they are generally in granular or powder form.

In the practice of the process of the present invention, a powdercomprising fine particles of porous structure and a food materialpredominantly comprising a starchy material and/or a protein as desired,and, if necessary, seasonings (e.g., sugar, salt and cheese), flavors orspices, colorants, edible oils and pure water in an appropriate amountshould first be prepared as starting materials.

A suitable quantity of the powder comprising fine porous particles isabout 1 to 20% of the total quantity of the starting materials, forexample, in the case of eggshell powder, and similar quantities mayapply to powders of other sources. The powder in an excessively smallquantity is not sufficiently effective in imparting a soft mouth feelwhile an excess of the powder tends to give a product which is too softand unpalatable. Generally, a quantity of about 1 to 10% is preferred.

The total water content in the starting materials fed to an extruder maybe about 10 to 18%, at which the materials can be easily swollen, in thecase where an increase in swelling percentage is generally intended inorder to produce swollen foods having a soft mouth feel and isordinarily below 25%. However, in order to also achieve the secondobject of this invention at the same time, water contents of about 25%or higher are required so that the end product, when used as a garnishfor soup, will not turn mushy in a short time but can remain crispy. Ifthe water content is lower than 25% of the total weight of the startingmaterials, they tend to be swollen excessively when they are extrudedand swollen through an extruder, and the end product, when used as agarnish for soup, will turn mushy in the mouth in a short time (20 to 30seconds). If the water content is excessive, for example, higher than50%, the swollen product obtained by extrusion will have a hard texturepresumably because the product cannot be easily made porous.

Even in the case where the total water content in the starting materialsis 25% or higher, however, the end product, while remaining crispy, willbecome too hard as a whole and feel hard in the core unless pulverizedeggshells or bones are added. In order to attain the second object ofthe present invention, the water content is preferably in the range offrom 30 to 45%.

Next, the starting materials described above are fed simultaneously in agiven quantity to an extruder through which they are heated andpressurized. The extruder used in the process of this invention may beof any type provided that it can heat and pressurize the startingmaterials: either of the uniaxial type or of the biaxial type. From thestandpoint of easy passage of kneaded materials through the extruder,however, a biaxial extruder which can be applied to a wider range ofwater content (8 to 95%) is preferable to a uniaxial extruder (10 to30%).

All of the starting materials may be fed to the extruder from a feederprovided at the starting or upstream end of the screw, or a part of thematerials (e.g., water) may be fed from an intermediate part of thebarrel covering the screw. Further, the starting materials may either bekneaded together before feeding or be fed at a rate sufficient to obtaina constant ratio of the respective starting materials without kneading.

The starting materials are heated by means both of the frictional heatgenerated during the passage of the starting materials through thebarrel of the extruder and of a heater (100° to 200° C.) provided at thebarrel.

The starting materials are pressurized as they are forced to passthrough the extruder by the screw.

The food material predominantly comprising a starchy material such ascereals is cooked by heating to 100° C. or higher temperatures wherebythe starch component thereof turns pasty (α-starch). The heatingtemperature ordinarily is about 100° to 180° C., preferably about 105°to 150° C. By heating at a temperature in this range, the starchcomponent turns sufficiently pasty while, on the other hand, the foodmaterial predominantly comprising a protein turns fluid so that thekneaded materials, when extruded through the extruder, are swollen underatmospheric pressure and made porous at the same time. Since the heatingtemperature is 100° to 180° C., the pressure within the extruder ismaintained ordinarily at about 1 to 40 kg/cm². The time for the foodmaterials to pass through the extruder depends on the temperature,pressure and other conditions but may generally be in the range of fromabout 15 seconds to about 6 minutes.

The starting materials are thus heated and pressurized through theextruder to convert the starchy material into α-starch, or to convertthe protein into a molten state by thermal denaturation, and further touniformly disperse fine particles of porous structure in the foodmaterials. The resulting molten starting materials are then forced outabruptly into the air from the die portion of the extruder to obtain aswollen product. The swollen product thus obtained is soft andsusceptible to deformation while the temperature thereof is high but,upon cooling in the air for a while, can be solidified into a desiredform such as a stick or a sheet according to the shape of the die of theextruder, and the resulting swollen product is generally provided as theend product.

In order to attain the second object of the present invention, however,a food material predominantly comprising a starchy material isprincipally used as a starting material, and the total water content inthe starting materials is maintained at 25% or higher, whereupon theextrudate obtained by the above procedure is cut to an appropriatelength and the so cut pieces of the extrudate are then heated and dried.Also in this case, the starting materials are extruded through the dieportion of the extruder as set forth above. The shape of the crosssection of the extrudate is determined by that of the die, but, when thetotal water content in the starting materials fed to the extruder or thekneaded starting materials is lower than 25%, especially about 5 to 15%,the materials are swollen to such a great extent at the outlet of theextruder (die portion) that the shape of the cross section of theextrudate is not dependent on that of the die but is substantiallycircular so as to minimize the cross-sectional area thereof, and furtherthe surface of the extrudate is so uneven that the extrudate of adefinite shape is difficult to obtain. The shape of the cross section ofthe die is suitably selected depending upon the desired shape of theextrudate cross section, for example, a square of sizes of 5 to 8 mm, acircle having a diameter of 5 to 6 mm or a heart shape.

The extrudate is then cut to a size, for example, of 5 to 20 mm inaccordance with the desired form of the end product. While anyappropriate cutting means may be employed, automatic or hand-operatedknives are generally used.

Heat-drying methods are not particularly limited, and hot air-drying orfrying is applicable in general. In a method of drying by hot air, forexample, hot air at 80° to 90° C. is used, and heating is carried outfor about 20 to 40 minutes. From the standpoint of flavoring the productobtained, heat-drying by frying is preferred. The frying conditions mayvary slightly with the moisture content of the extrudate, but frying ata temperature of 150° to 190° C. for about 30 seconds to 5 minutes isgenerally suitable.

The starting materials heated and pressurized into a molten state in theextruder are extruded into the air through the die portion thereof,whereby the air and moisture contained in the molten starting materialsexpand explosively all at once to form a swollen product.

In accordance with the process of the present invention, fine particlesof porous structure are uniformly dispersed in the molten startingmaterials, and it is presumed that these fine particles serve to lessenthe sizes of the foam bubbles generated in the swollen product when thestarting materials are swollen and also disperse the foam bubblesuniformly in the swollen product.

As is apparent from the results of Test Example 1 which will be setforth hereinlater, the cells or foam bubbles formed in the swollen foodobtained by the process of the present invention are smaller than thoseformed according to the conventional process and are dispersed uniformlyin the swollen food. For this reason, a swollen food having a densesurface and cross section can be obtained by the process of the presentinvention. Further, small cells result in thin cell membranes, wherebythe whole swollen food can be made soft.

By applying the process of the present invention, it is thereforepossible to facilitate the production of swollen foods for infantsincluding baby foods for which a soft mouth feel is desired.

In the case where a food predominantly comprising a starchy material isused as the food material and eggshell powder or bone powder is used asthe fine particulate powder of porous structure while the total watercontent in the starting materials, including the above mentioned foodmaterial, fed to the extruder is maintained at 25% or higher, and theextrudate is cut to an appropriate length, heated and dried inaccordance with the process of the present invention, a swollen cerealfood obtained as the end product is not too soft or too hard and feelsadequately crispy when used in various ways, for example, as a garnishfor soups. Furthermore, this food can be suitably crushed by chewing anddoes not feel hard in the core when used as a garnish for a vegetablesalad and eaten with a dressing, and hence it can be used in a widerrange of foods.

The swollen food which has achieved the third object of the presentinvention will be described in detail in Test Example 3 set forthhereinlater.

EXPERIMENTAL EXAMPLES

The present invention will now be described in more detail withreference to test examples and specific examples of practice wherein allpercentages are "% by weight".

Test Example 1

Eggshell powder (maximum particle size: 20 μ), cornstarch and pure waterwere prepared as starting materials, the quantity of the pure water usedbeing 6% of all the starting materials, and the ratio of the eggshellpowder to the cornstarch used being as shown in TABLE 1. Subsequently,all the starting materials of the respective compositions were heated,pressurized and swollen through an extruder under the followingconditions to obtain 6 types of swollen food samples of differentcompositions of starting materials.

For each of the samples thus obtained, the apparent bulk density wasmeasured, and the mouth feel of the sample was tested, while the colortone of the sample and the state of swollen cells were respectivelyobserved. The results obtained are shown in TABLE 1. It is apparent fromthese results that the swollen food comprising eggshell powder had asofter mouth feel than that comprising no eggshell powder.

                                      TABLE 1                                     __________________________________________________________________________            Sample     1        2      3      4     5     6                       __________________________________________________________________________    Composition of                                                                        Quantity of eggshell                                                                     0        2      5      10    20    40                      starting                                                                              powder (%)                                                            material                                                                              Quantity of cornstarch                                                                   94       92     89     84    74    54                              (%)                                                                   Properties                                                                            Apparent bulk density                                                                    0.22     0.09   0.07   0.09  0.09  0.08                    of product                                                                            Mouth feel hard     soft   soft   soft  soft  soft but                (swollen food)                                        unpalatable                     State of swollen cell                                                                    coarse (ca. 8-4                                                                        fine   fine   fine(-) &                                                                           fine(-)                                                                             microscopic(-)                             mm) & ununiform                                                                        (ca. 2-1 mm)                                                                         (ca. 1-0.5                                                                           uniform                                                                             uniform                                                                             & uniform                                           & uniform                                                                            & uniform                                          Color tone Pale yellow                                                                            White  White  White White Slightly                __________________________________________________________________________                                                          gray                

Notes:

(1) The figures in the parentheses roughly show the range of the size ofthe cells, i.e., foam bubbles.

(2) Extruding conditions:

All of the starting materials were fed into a biaxial extruder (suppliedby Toshiba Kikai K.K., Model "TEM-50B", screw rotational speed 150 rpm,barrel temperature 140° C.) simultaneously in a given quantity andpressurized at 20 to 30 kg/cm² under heating to convert the materialsinto a molten state.

Subsequently, the molten starting materials were discharged into the airfrom the die portion of the extruder to swell the same to obtain aswollen food. The swollen product thus obtained was left standing tocool for 5 minutes at room temperature. Thereafter, the mouth feel andother characteristics of the product were determined.

(3) Similarly, a swollen product was prepared by the above procedurefrom the starting materials comprising 1% of eggshell powder, 93% ofcornstarch and 6% of pure water. The mouth feel and othercharacteristics of this product were rated substantially between thoseof Sample 1 and Sample 2.

(4) A similar test was conducted with a sample in which the cornstarchin the starting materials was replaced by soybean protein powder, andthe effect of softening the mouth feel of the product due to theaddition of eggshell powder was observed to be substantially similar tothat in the case where cornstarch was used.

Example 1

10 kg of eggshell powder (maximum particle size 20 μ) 90 kg ofcornstarch and 6 kg of pure water were fed into a biaxial extruder(supplied by Toshiba Kikai K.K., Model "TEM-50B", screw rotational speed150 rpm, barrel temperature 140° C.) simultaneously in a given quantityand pressurized at 20 to 30 kg/cm² under heating to convert thematerials into a molten state.

Subsequently, the molten starting materials were discharged into the airfrom the die portion of the extruder to swell the same. The swollenproduct thus obtained was left standing to cool for 5 minutes at roomtemperature, whereby 95 kg of a swollen food having dense surfaces andmoreover a soft mouth feel was obtained.

Example 2

6 kg of bovine bone powder (maximum particle size 50 μ), 95 kg of flour(high-gluten flour), 1 kg of table salt, 2 kg of sugar, 0.1 kg of yeastextract and 6 kg of pure water were fed into a biaxial extruder (screwrotational speed 120 rpm, barrel temperature 140° C.) simultaneously ina given quantity and pressurized at 20 to 30 kg/cm² under heating toconvert the materials into a molten state.

The resultant molten starting materials were discharged into the airfrom the die portion of the extruder to swell the same. The swollenproduct obtained was left standing to cool for 5 minutes at roomtemperature, whereby 92 kg of a snack confection having a dense crosssection surface and a soft mouth feel was obtained.

Example 3

A swollen product was produced as in Example 1 except that thecornstarch was replaced by soybean protein (powder), whereby a swollenfood having dense surfaces and moreover a soft mouth feel was obtained.

Test Example 2

In all of the samples of the compositions shown in the following TABLE,the starting materials excluding water were fed into a biaxial extruder(screw rotational speed 280 rpm, barrel temperature 130° C, barrelinternal pressure ca. 30 kg/cm²) from a feeder at a rate of 0.42 kg/min.while water was fed through an inlet provided at the barrel portion. Thestarting materials in entirety were kneaded and heated in the barrel,and discharged into the air through a die (of approximately 6 × 6 mmsquare cross section). For each sample, the kneaded materials werepassed through the extruder for approximately 2 minutes.

Each of the extruded samples (in the form of a bar) was manually cutwith a knife at intervals of 6 mm, and the so cut pieces were fried inoil at 170° C. to produce croutons.

The mouth feel of the croutons thus produced from each sample wasexamined, whereupon the results shown in TABLE 2 below were obtained.

As is apparent from the data in TABLE 2, the croutons obtained by theprocess of the present invention are excellent in crispiness and shapeof the product as compared with those obtained by the control processes.

                                      TABLE 2                                     __________________________________________________________________________    Conditions         Process of the invention                                                                     Control process I                                                                             Control process             __________________________________________________________________________                                                      II                          Composition of starting material                                              Flour (moisture content: ca. 14%)                                                                68(%)          70(%)           91(%)                       Eggshell powder (average particle size:                                                          4              0               4                           2-3μ)                                                                      Water              28             30              5                           Total water content in starting materials                                                        38             40              18                          Heating-drying condition                                                      Frying temperature &                                                                             170° C. 170° C.  170° C.              time               2 min.         2 min.          1 min. 20 sec.              Moisture content in fried crouton                                                                2.0%           2.2%            1.5%                        Properties of product                                                         Mouth feel of product sprinkled over hot                                                         Does not turn mushy and is                                                                   Does not turn mushy but                                                                       Turns mushy                 soup as determined after 3 min.                                                                  adequately crispy                                                                            hard in the core                            Mouth feel of product itself                                                                     Adequately crispy, easily                                                                    Hard as a whole and in the                                                                    Easily crushed and                             crushed and yet soft to the                                                                  (so hard as to feel prickly in                                                                yet soft to the core                           core           mouth)                                      Shape of product   Substantially dice-shaped                                                                    Substantially dice-shaped                                                                     Substantially                                                                 cylindrical                                                                   (with no angles                                                               character-                                                                    istic of dice)              Apparent bulk density                                                                            0.35           0.36            0.11                        Size of cell       ca. 0.5-0.1 mm ca. 1-0.5 mm    ca. 1-0.5                   __________________________________________________________________________                                                      mm                      

Test Example 3

In all of the samples of the compositions shown in the following TABLE,the starting materials excluding water were fed into a biaxial extruder(screw rotational speed 280 rpm, barrel temperature 130° C., barrelinternal pressure ca. 30 kg/cm²) from a feeder at a rate of 0.42 kg/min.while water was fed through an inlet provided at the barrel portion. Thestarting materials in entirety were kneaded and heated in the barrel,and discharged into the air through a die (of approximately 6x6 mmsquare cross section). For each sample, the kneaded materials werepassed through the extruder for approximately 2 minutes.

Each of the extruded samples (in the form of a bar) was manually cutwith a knife at intervals of 6 mm, and the so cut pieces were heatedwith hot air at 80° to 90° C. for 20 to 30 minutes so that the finalmoisture content of the product would be 2.0%, thereby to producecroutons.

The mouth feel of the croutons thus produced from each sample wasexamined. The results obtained are summarized in TABLE 3.

As is apparent from the data in TABLE 3, the product crouton (swollencereal food) having an apparent bulk density of about 0.22 to 0.39 and acell size of about 0.7 to 0.08 mm remains appropriately crispy whensprinkled onto a hot soup and yet is not too hard.

It will also be evident that the crouton produced can retain the shapeof the die relatively faithfully.

                                      TABLE 3                                     __________________________________________________________________________                    Sample                                                        Conditions      1     2     3     4     5     6     7     8                   __________________________________________________________________________    Composition of starting material                                              Flour (moisture content: ca. 14%)                                                             88.3(%)                                                                             82.5(%)                                                                             76.7(%)                                                                             70.9(%)                                                                             65.1(%)                                                                             59.3(%)                                                                             53.4(%)                                                                             Com-                Eggshell (average particle size:                                                              4     4     4     4     4     4     4     mercially                                                                     -2-3μ)                                                                     4available          Water           7.7   13.5  19.3  25.1  30.9  36.7  42.6  crouton             Total water content in starting                                                               20    25    30    35    40    45    50    (prepared           materials                                                 from                                                                          bread)              Drying under heating                                                          Hot air temperature and                                                                       80-90° C.                                                                    "     "     "     "     "     "     Com-                blowing time    20-30 min                                 mercially                                                                     available                                                                     crouton                                                                       (prepared                                                                     from                                                                          bread)              Moisture content in dried crouton                                                             2.0%  2.0%  2.0%  2.0%  2.0%  2.0%  2.0%  2.0%                Properties of product                                                         Mouth feel of product sprinkled                                                               X.sup.(1)                                                                           Δ.sup.(1)                                                                     ○                                                                            ○                                                                            ○                                                                            ○                                                                            Δ.sup.(2)                                                                     X                   over hot soup as determined                                                   after 3 min. (mouth feel (1))                                                 Mouth feel of product itself                                                                  ○                                                                            ○                                                                            ○                                                                            ○                                                                            ○                                                                            ○                                                                            Δ                                                                             ○            (mouth feel (2))                                                              Shape of product                                                                              X     Δ                                                                             ○                                                                            ○                                                                            ○                                                                            ○                                                                            ○                                                                            ○            Apparent bulk density                                                                         0.10  0.16  0.22  0.28  0.34  0.39  0.45  --                  Size of cell    ca. 1-0.5                                                                           ca. 0.9-                                                                            ca. 0.7-                                                                            ca. 0.6-                                                                            ca. 0.5-                                                                            ca. 0.3-                                                                            ca.                                                                                 ca. 1-0.5                           mm    0.4 mm                                                                              0.3 mm                                                                              0.2 mm                                                                              0.1 mm                                                                              0.08 mm                                                                             0.05                                                                                mm                  __________________________________________________________________________

Notes:

(1) The symbols O, Δ and X in the TABLE respectively indicate thefollowing characteristics.

Mouth feel (1):

X.sup.(1) softens to turn mushy in 3 to

5 seconds

Δ.sup.(1) turns mushy in 0.5 to 1 minute

O remains moderately hard and feels crispy for about 3 minutes

Δ.sup.(2) remains moderately hard and feels crispy for about 3 minutes,but feels hard in the core (too hard)

Mouth feel (2):

O feels soft to the core (appropriately crushed)

Δ feels slightly hard in the core (not easily crushed)

Shape of product:

X substantially cylindrical (with no angles characteristic of dice)

Δ while having angles, each edge expanding outwardly in an arch form(swollen)

O substantially dice-shaped

(2) The sizes of the cells in the extrudate were measured with respectto those visible in a cross section perpendicular to the direction ofextrusion. Few cells were found on the outer sides of the extrudate inthe direction of extrusion.

Example 4

Croutons were produced in accordance with the process of the presentinvention described in Test Example 2 above except that the startingmaterials were used in the following proportions.

    ______________________________________                                        Starting material      Proportion (%)                                         ______________________________________                                        Flour (moisture cont.: ca. 14%)                                                                      64                                                     Eggshell powder (av. particle size: 6μ)                                                           4                                                      Soybean protein powder 4                                                      (mositure cont.: ca. 6%)                                                      Water                  28                                                                            100                                                    ______________________________________                                    

The water content in the overall starting materials was approximately37%, and the moisture in fried croutons was approximately 1.8%.

Example 5

Croutons were produced in accordance with the process of the presentinvention described in Test Example 2 above except that the startingmaterials were used in the following proportions.

    ______________________________________                                        Starting material     Proportion (%)                                          ______________________________________                                        Flour (moisture cont.: ca. 14%)                                                                     65                                                      Bone powder (av. particle size: 6μ)                                                              4                                                       Shortening            1                                                       Sugar                 2                                                       Water                 28                                                                            100                                                     ______________________________________                                    

The water content in the overall starting materials was approximately37%, and the moisture in fried croutons was approximately 1.8%.

Example 6

Croutons were produced as in the process of the present inventiondescribed in Test Example 2 above except that the starting materialswere used in the following proportions and that the following extrudingconditions were employed.

    ______________________________________                                        A: Starting material   Proportion (%)                                         ______________________________________                                        Flour (moisture cont.: ca. 14%)                                                                      64                                                     Eggshell powder (av. particle size: 10μ)                                                          3                                                      Water                  33                                                                            100                                                    ______________________________________                                    

The water content in the overall starting materials was approximately42%, and the moisture in fried croutons was approximately 1.7%.

    ______________________________________                                        B: Extruding conditions                                                       ______________________________________                                        Screw rotational speed 250 rpm                                                Barrel temperature     120° C.                                         Barrel internal pressure                                                                             ca. 35 kg/cm.sup.2                                     Starting material feeding rate                                                                       0.60 kg/min.                                           ______________________________________                                    

Kneaded materials were passed through the extruder for approximately 2minutes.

Example 7

Croutons were produced as in the process of the present invention asdescribed in Test Example 2 except that the extrudate cut at intervalsof 6 mm was heat-dried with hot air instead of frying (temperature: 90°C., time 20 minutes). The moisture content in the end product obtainedwas approximately 3.5%.

None of the croutons obtained in Examples 4 through 7 describedhereinbefore turned mushy in the mouth, but they remained crispy even 1to 2 minutes after they were sprinkled onto a hot soup as a garnish.

We claim:
 1. A process for producing swollen foods which comprisesheating and pressurizing through an extruder a powder comprisingeggshell powder having a maximum particle size of 100 μ and a foodmaterial predominantly comprising a starchy material or a protein ormixtures thereof and thereafter releasing said powder and food materialthereby causing the same to swell.
 2. A process as claimed in claim 1,wherein 1 to 20%, based on the total quantity of the starting materials,of eggshell powder is used.
 3. A process as claimed in claim 1, whereina food predominantly comprising a starchy material is used as the foodmaterial.
 4. A process as claimed in claim 3 wherein the total watercontent in the starting materials fed to the extruder is maintained at25% or higher and the resulting extrudate thus caused to swell is cut toan appropriate length, heated and dried.
 5. A process as claimed inclaim 4, wherein the product obtained after heating and drying is acrouton.
 6. A process as claimed in claim 4, wherein the extruder is ofthe uniaxial or biaxial type.
 7. A process as claimed in claim 4,wherein the drying method involves through-flow drying or frying in oil.